Various pneumatic tires have been proposed which contain a built-in sealant layer based upon a organoperoxide depolymerized butyl rubber layer. For example, see U.S. Pat. Nos. 4,895,610, 4,228,839, 4,171,237 and 4,140,167 and U.S. patent application Ser. Nos. 10/171,057, 10/368,259 and 2005/0205186.
Additional patent publications which propose various tire constructions which may involve built-in or built-on sealants for tires such as for example, U.S. Pat. Nos. 1,239,291, 2,877,819, 3,048,509, 3,563,294, 4,206,796, 4,286,643, 4,359,078, 4,444,294, 4,895,610, 4,919,183 and 4,966,213.
In one embodiment, the built-in sealant layer may be derived from butyl rubber which contains precipitated silica with minimal, if any, rubber reinforcing carbon black and have an identifying color other than black.
In practice, the organoperoxide compound yields free radicals at an elevated temperature which operate to depolymerize the butyl rubber of the built-in sealant precursor composition within the tire itself to form the built-in sealant layer.
For this invention, it is considered herein to be important that the butyl rubber of the sealant composition be depolymerized by the organoperoxide generated free radicals to an extent that the modulus (G′) for the resultant sealant layer is reduced to a value of 40 kPa or less, alternately and usually more desirably less than 26 kPa, (100° C., 1 Hertz, 5 percent strain), often desirably with a minimum of about 10 kPa. Accordingly, while a broad range of from about 10 to about 40 kPa might be useful for some tires, a more narrow range of from about 10 to about 28 kPa, or from about 16 to about 28 kPa might be desired, depending upon the tire itself, including tire size and intended tire service conditions, to promote sealant efficiency of the built-in tire sealant, namely to promote an ability to efficiently seal against various puncturing objects such as, for example, a nail.
Various individual organoperoxide compounds have heretofore been proposed for the depolymerization of the butyl rubber of the tire built-in sealant precursor layer including, for example, individual organoperoxides such as dicumyl peroxide and 4,4-di(tertiary butylperoxy) valerate.
However, in contrast, an entirely different manipulative approach is proposed as compared to a more simple use of an individual organoperoxide or simple mixture of organoperoxides to effect the formation of the depolymerized butyl rubber based built in sealant for a tire.
In practice, use of dicumyl peroxide for the silica-containing butyl rubber depolymerization has been considered as being desirable because, while it requires a higher temperature to effectively activate its free radical formation, an advantage to using dicumyl peroxide is that its byproducts tend to be primarily relatively high boiling point products which are liquid at room temperature (e.g. at about 23° C.), such as, for example, cumyl alcohol.
In practice, use of 4,4-di-(tertiary butylperoxy) valerate has been considered as being desirable for the silica-containing butyl rubber depolymerization of the tire because it tends to form the necessary free radicals at a lower temperature than that of the dicumyl peroxide. However, its byproducts tend to be significantly lower boiling point products such as, for example t-butyl alcohol.
Various decomposition related information for the dicumyl peroxide and 4,4-di(tertiary butylperoxy) valerate is provided in the following Table A.
TABLE ASDAT, selfActivationHalf LifeacceleratedHalf LifeEnergyat 150° C.,decompositionTemperatureKcal/moleminutesTemp. ° C.1 hour, ° C.Dicumyl371593137peroxide4,4-di(tertiary356.575129butylperoxy)valerate
It is seen from Table A that while the activation energies for the dicumyl peroxide and the di(tertiary butylperoxy) valerate are similar, the indicated temperature at which the organoperoxide is initially significantly involved with its substantive decomposition in a sense of forming free radicals to promote decomposition of the butyl rubber is significantly lower for the di(tertiary butylperoxy) valerate, namely about 75° C., as compared to the higher temperature of about 93° C. for the dicumyl peroxide.
Therefore, as the rubber composition's temperature increases within the hot mold from, for example, about 23° C. to an ultimate temperature in a range of from about 150° C. to about 170° C., the very small amount of di(tertiary butylperoxy) valerate might initiate its substantive free radical generation significantly early in time, as the rubber composition's temperature approaches about 75° C., to initiate a beginning of a depolymerization of the butyl rubber which may occur before a substantive free radical generation by the dicumyl peroxide as the rubber composition's temperature proceeds to increase and approach a higher temperature of about 93° C.
Taking into account the significantly shorter half life of the di(tertiary butylperoxy) valerate free radical initiation, (reported half life of about 6.5 minutes as compared to about 15 minutes for the dicumyl peroxide), it is possible that an extended continuation of depolymerization of the butyl rubber might be largely dependent upon the extended free radical generation activity of the dicumyl peroxide.
Therefore, the 4,4-di(tertiary butylperoxy) valerate might be significantly more active in a sense of having a considerably greater free radical formation rate, and therefore a considerably greater promotion of butyl rubber depolymerization over a shorter time, than the dicumyl peroxide.
The 4,4-di-(tertiary butylperoxy) valerate might therefore be a favored organoperoxide to initiate depolymerization of the butyl rubber because it not only begins its free radical formation at a significantly lower temperature than the dicumyl peroxide, but it apparently has a greater rate of free radical formation.
It is to be appreciated that the presence of the silica in the butyl rubber based sealant precursor layer complicates the butyl rubber depolymerization process both in the sense that the silica reacts with (for example by its hydroxyl groups) the organoperoxide to form byproducts from the organoperoxide decomposition as well in a sense that adsorbing of the organoperoxide onto the precipitated silica thereby tends to inhibit or retard the rate and degree of depolymerization of the butyl rubber of the sealant precursor.
Accordingly, it is therefore a significant undertaking of this invention to evaluate and determine if the contrasting properties of the respective organoperoxides can be utilized in a way to enhance depolymerization of the butyl rubber in the sealant precursor composition in situ within a tire configuration to ultimately form the built-in silica-containing sealant layer.
For such evaluation, it is to be appreciated that the depolymerization of the butyl rubber in the sealant precursor is to be accomplished somewhat within the time and temperature cure conditions of the tire in which the built-in sealant is formed. It is to be recognized that the time and temperature for the curing of the tire may vary somewhat depending upon nature of the tire itself which may include, for example, the size of the tire. For evaluation purposes in the laboratory, a time and temperature for the depolymerization of the butyl rubber in sealant precursor composition to reach an appropriate modulus (G′) value may be used to somewhat approximate the time and temperature, or an average time and temperature, for a typical tire cure condition.
Such determination is to further evaluate if a greater control over the degree and rate of depolymerization of the silica-containing butyl rubber can be accomplished to more effectively convert the sealant precursor to the built-in sealant and to achieve a suitable modulus (G′) for the sealant composition use of the combination organoperoxides.
While both organoperoxides have heretofore been proposed for depolymerization of butyl rubber for a tire built-in sealant, the aforesaid manipulative combination of free radical initiation and associated depolymerization initiation combined with cooperative propagation of continued free radical initiation and associated propagation of continued butyl rubber depolymerization is a significant aspect of this invention, and is not an obvious manipulative combination without a trial and error evaluation and is a significant departure from past practice.
A further embodiment of the invention, in combination of the aforesaid specified combination of organoperoxides with differentiating activation temperatures, is a treatment of the precipitated silica with polyethylene glycol, prior to the organoperoxide addition to the rubber composition, in order to inhibit, retard and/or significantly prevent significant contact of hydroxyl groups contained on the precipitated (synthetic amorphous) silica aggregates by the combination of the aforesaid organoperoxides.
Accordingly, in one embodiment, the precipitated silica may be treated in situ within the rubber composition by the polyethylene glycol prior to addition of the organoperoxide combination, or, in another embodiment, may be pre-treated by the polyethylene glycol prior to addition of the silica to the rubber composition. The polyethylene glycol is a low molecular weight polyalkylene oxide polymer, which might sometimes be referred to as a polyalkylene glycol (e.g. polyethylene glycol).
Indeed, it is considered herein that significant challenges are presented using the precipitated silica (optionally also including the clay when used in combination with the precipitated silica), particularly when used in place of rubber reinforcing carbon black for reinforcing filler for a non-black colored sealant for the above reasons.
Therefore, as indicated above, when the precipitated silica is used, it is preferably treated with a polyalkylene glycol (e.g. polyethylene glycol).
In a further embodiment of the invention, while the butyl rubber, as a copolymer of isobutylene and isoprene, may be composed of greater than one weight percent units derived from isoprene, it is preferred that it is composed of from only about 0.5 to 1.0 weight percent units derived from isoprene. The use of a butyl rubber with such low unsaturation content is to promote a more efficient depolymerization by treatment with the organoperoxide where it is envisioned that the presence of the double bonds within the butyl rubber may tend to terminate its depolymerization when the depolymerization process reaches the double bond unsaturation in the butyl rubber.
In an additional embodiment of the invention, to promote better processing of the butyl rubber-based sealant precursor composition, it is desired to use a butyl rubber that has a relatively high Mooney viscosity (ML+8) value at 125° C. in a range of from about 25 to about 60, alternately from about 40 to about 60.
Thus a butyl rubber of very low isoprene-based unsaturation content (for more effective depolymerization of the butyl rubber) and relatively high Mooney viscosity (to promote better physical handling of the sealant precursor composition) is a desirable combination.
In practice, it is desired herein for the butyl rubber-based sealant precursor composition to have a modulus (G′) physical property, (at a 5 percent dynamic strain at 100° C. and 1 hertz) in a range of about 170 to about 350 kPa, alternately in a range of from about 175 to about 300 kPa.
For such purpose, it is desired herein for the depolymerized butyl rubber sealant composition to have a significantly lower storage modulus (G′) physical property as heretofore indicated.
In practice, such modulus (G′) may be determined, for example, by an RPA (Rubber Process Analyzer) instrument which measures the strain sweep at 100° C. at 1 Hertz over a range of, for example, from 1 to 50 percent strain. Such storage modulus (G′) measurement for rubber samples is well known to those having skill in such art. Such a Rubber Process Analyzer is RPA 2000™ instrument by Alpha Technologies, formerly the Flexsys Company and formerly the Monsanto Company. References to an RPA-2000 instrument may be found in the following publications: H. A. Palowski, et al, Rubber World, June 1992 and January 1997, as well as Rubber & Plastics News, Apr. 26 and May 10, 1993.
In the description of this invention, the term “phr” is used to designate parts by weight of an ingredient per 100 parts of elastomer unless otherwise indicated. The terms “elastomer” and “rubber” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.